A shape of a mist injected by an injection valve, a particle size of the mist and so on are important in an engine, in view of an unburnt fuel or a delay of vaporizing. Therefore, a mist from an injection valve is tested in a mass production line. There are various methods for the testing of a mist. For example, there are methods of measuring a particle size of a mist particle by optically taking an image of a mist such as an optical diffraction, a PDPA (Phase Doppler Particle Analyzer), a patternator, a laser holography, a laser shadow or the like. There is a method of testing a mist by radiating a light and using a scattered light. For example, Patent Document 1 discloses a method of measuring the number of micro particles with use of an intensity of the scattered light.